This invention relates generally to magnetic field-tunable microwave filters and in particular to such filters utilizing ferrimagnetic resonator elements, such as of yttrium-iron-garnet (YIG).
One type of known YIG filter utilizes a single piece housing in which to mount YIG spheres, coupling loops, coaxial cables, and their associated mounting parts. With this approach, relatively large holes, in comparison to a typical coupling loop wire diameter, are drilled through the housing in order to permit coaxial cables to be fed to the YIG sphere cavity or cavities in the housing. If the transition from the coaxial cable center conductor to a coupling loop is made at the cavity edge, the coupling to spurious magnetostatic modes is greatly increased. If, on the other hand, the transition is made too far away from the edge, inductance is added to the filter which causes the input coupling to change as the filter frequency is tuned. These single piece housing microwave filters are limited to relatively low maximum tuning frequencies.
U.S. Pat. No. 4,334,201 of Shores discloses a YIG band pass filter having a housing which is split into two sections or rings. The two section technique of the Shores patent facilitates the placement of small passages and holes within the housing. These small holes accommodate the various components of the filter. This patent specifically discloses molding the housing components out of powdered metal, such as German Silver. Without providing any further details, the Shores patent mentions that any other suitable material and process may be used to fabricate the housing rings. Filters formed of molded powdered metallic material have been tunable in general only to about 2 to 20 Gigahertz. To significantly extend the frequency range using molded powdered metal is not believed possible. To extend the frequency range using powdered metal and machining, instead of molding, would require an extremely skilled machinist. An filter manufacturing technique which relies upon the skill of a machinist does not reliably result in tunable filters with consistent performance characteristics.
In devices made in accordance with U.S. Pat. No. 4,334,201, metallic shims are used to cover and close the YIG sphere containing cavities. These shims are gold plated and conform to the shape of magnet pole piece receiving recesses found in the components of the filter housing. Respective pole pieces of a magnet are pressed against these shims to hold them in place and close the YIG resonator cavities. As a result, the magnetic material forming the pole pieces is placed under mechanical stress, which interferes with tuning linearity of the filter, especially at higher frequencies. Therefore, tuning of these filters is unpredictable. In addition, shims of this type provide limited sealing of the ends of the YIG filter sphere cavities. Also, in these devices the gap between the pole pieces across the shims and housing components was about 0.065 inches.
Filter housing components have also been formed by injection molding of plastic and coating these plastic parts with metal and by injection molding of metal. Again, the maximum frequencies to which such filters have been tunable are about the same as have been achieved using molded powdered metal components.
Therefore, a need exists for an improved microwave filter of this type, and for a method of manufacturing such a filter, which is directed towards overcoming these and other disadvantages of the prior art.